The present invention is directed to a method of disconnecting metallization in a polymeric dielectric electrical interconnect, such as a substrate or printed circuit board, and more particularly to reworking short-circuited connections or customization severing of connections between copper lines in high density copper/polyimide substrates.
Advances in integrated circuit miniaturization have led to high density electrical interconnection networks that accommodate high density packaging. A polymeric material layer, such as a polyimide layer, can be formed over an electrically conductive metallization pattern in sufficient thickness to act as a dielectric layer. Copper is a preferred electrical conductor with polyimide, and can be fabricated with densities of over 500 lines per inch per level. Copper/polyimide interconnects can be fabricated with either stacked or staggered vias, and can be in a predetermined circuit pattern or customizable. Good design practice requires getting the necessary interconnect in a minimum area for very short signal delays between chips, and in a minimum number of layers to reduce the cost and manufacturing complexity.
There is a need to disconnect the electrically conductive metallization in polyimide interconnects in a simple and convenient manner. This need may arise due to undesired short-circuits between adjacent metallization lines, or due to a customization process which tailors the metallization routing to a specific application.
Conventionally, the patterned etching of thin metallization is accomplished by the use of photolithographic masks, in which an etchant-resistant coating (resist) is laid down upon a substrate and selectively developed and removed. The disadvantages of organic resists are well known, such as contamination from organic solvents and degradation at temperatures over 200.degree. C.
To overcome these difficulties numerous maskless techniques have been developed. U.S. Pat. No. 4,619,894 describes patterning an aluminum oxide layer on a substrate by selective heating. U.S. Pat. No. 4,615,904 describes forming a thin absorptive layer on a substrate, prenucleating portions of the absorptive layer with a laser, and depositing material on the prenucleated portions. U.S. Pat. No. 4,622,095 describes exposing a metallized substrate to a laser in a halogen gas atmosphere to form a metal halide salt reaction product on the substrate.
The use of an excimer laser to remove selected regions of a resist by way of photoablation in order to pattern the underlying layer is known in the art. It is termed "ablative photodecomposition" (APD) and requires high powered pulsed sources. U.S. Pat. No. 4,414,059 describes an APD process wherein ultraviolet radiation of wavelengths less than 220 nm cause fragmentation of resist polymer chains and the immediate escape of the fragmented portions from the resist layer. In effect, the incident energy is transferred to kinetic energy in rapid and large amounts of bond breaking whereby the polymer chains are fragmented and explode-off and escape as volatile by-products. No subsequent development step is required for patterning the resist layer. The '059 patent, however, is restricted to the use of a resist layer for lithography in which after the lithography is finished, the photoablated resist layer is completely removed and the device fabrication procedure continues.
Likewise, U.S. Pat. No. 4,568,409 describes a process for precision separation of layers, in which the layer to be separated is coated with a functional material that is selectively removed by spectral energy to form a mask which is resistive to an etchant chosen to etch underlying material. The '409 patent also teaches the removal of the functional material after etching by dissolution in a suitable solvent.
Another APD technique is described in U.S. Pat. No. 4,780,177 wherein a thin layer of photoablative polymer disposed on a metal layer is covered with a thick transparent material. A beam of laser energy is directed through the upper layer and is absorbed by the lower layer. The ablated lower layer also blows off the thick transparent upper layer resulting in higher etch resolution capability.
The use of ultraviolet radiation for etching polyimide is also known in the art. U.S. Pat. No. 4,508,749 describes the use of a U.V. source between 240-400 nm to etch through a polyimide layer. The '749 patent is primarily directed to producing tapered openings in the polyimide structure between metallization layers so that metallization can then be deposited in the openings to connect metallic layers adjacent to the top and bottom surfaces of the polyimide.
Another approach is described in U.S. Pat. No. 4,684,437 in which a polymer/metal structure is irradiated in order to remove one material without harming another. Laser wavelength and energy fluence per pulse are selected so that the removal rate of the metal due to thermal processes is significantly greater than the removal rate of the polymer by APD.